1. Field of Industrial Utilization
The present invention relates to a glass antenna for automobiles which uses, as a part of the antenna, a defogging heater wire installed in the rear windshield and more particularly to an antenna which is a combination of the heater wire and a separately mounted antenna to receive FM and AM broadcasts, etc.
2. Prior Art
The antennas shown in FIGS. 6 and 7 are known as examples of conventional automobile glass antennas.
In the antenna shown in FIG. 6, a main antenna A which has an antenna output terminal is formed on the surface of window glass 10 as a separate element from a defogging heater wire H. Generally, main antennas are formed in an asymmetrical shape so that they are resonant in the FM frequency band at the most optimized reception and maintain the improved FM directionality. However, even if such a structure is taken, matching cannot be accomplished for the entire FM reception frequency band because the area which can be used as an antenna is small. As a result, the FM reception sensitivity is low, and the FM directionality cannot be improved sufficiently. In addition, AM reception sensitivity is also low. As a result, in order to improve the FM and AM reception sensitivities, an FM compensating amplifier 31 and an AM compensating amplifier 32 are used between the antenna output terminal and a feeder cable F.
In the conventional antenna illustrated in FIG. 7, an AM choke coil CHa and an FM choke coil CHfO are utilized. These coils are for blocking high-frequency signals at both terminals of the defogging heater wire H. The heater wire H is thus "insulated in terms of high-frequency" by the choke coils from power supply circuit B so that the heater wire H can be used as an antenna. As seen from the above, since the heater wire H is used as an antenna though it is originally not designed to be an antenna, matching cannot be obtained in the FM frequency band, and the FM reception sensitivity is low. On the other hand, since there is a large amount of stray capacitance for the AM frequency band, the capacitance splitting loss increases, which brings an AM reception sensitivity drop. As a result, in order to compensate for the poor FM and AM reception sensitivities, an FM compensating amplifier 31 and an AM compensating amplifier 32 are installed between the antenna output terminals and the feeder F.
In the above-described conventional antennas, a matching for the entire FM reception frequency band cannot be obtained if only the main antenna A or heater wire H is used, which results in FM reception sensitivity drop. This is the reason for using the FM compensating amplifier 31. When the FM compensating amplifier 31 is used, it is however necessary that such an amplifier 31 is a broad-band amplifier which can cover the entire FM reception frequency band. This in turn brings about noise and cross-modulation or inter-modulation in intense electric fields.
The object of the present invention is to provide a glass antenna for automobiles which has a good FM reception with a simple structure of a combination of a heater wire and a conductor.